1. Field of the Invention
The present invention relates to processes for forming filaments of a thermotropic liquid crystalline polymer. Specifically, the present invention provides processes for forming as-spun and heat-treated high denier filaments of a variety of thermotropic liquid crystalline wholly aromatic polyesters and polyesteramides. This invention also relates to as-spun and heat-treated high denier filaments of thermotropic liquid crystalline polyesters and polyesteramides.
2. Description of the Prior Art
Thermotropic liquid crystalline polymers (LCPs) are an important class of polymers, which are generally wholly aromatic molecules containing a variety of heteroatom linkages including ester and/or esteramide linkages. Upon heating to sufficiently high temperature, LCPs melt to form a liquid crystalline melt phase (often referred to as xe2x80x9canisotropic phasexe2x80x9d) rather than an isotropic melt. Generally, LCPs consist of linear (xe2x80x9crigid rodxe2x80x9d) molecules that can line up to yield the desired liquid crystalline order. As a result, LCPs feature low melt viscosity and thus improved performance and processabilities.
Because LCPs orient to form xe2x80x9crigid rodxe2x80x9d linear molecules, LCPs exhibit extremely high mechanical properties. Thus, it is well known in the art that LCPs can be formed into shaped articles, such as films, rods, pipes, fibers, and various other molded articles. In addition, it is also known in the art that LCPs, particularly in the fiber form, exhibit exceptionally high mechanical properties after a heat treatment process. However, all of the known methods in the art describe formation of only the low denier fibers, e.g., of about 10 deniers per filament (dpf), which exhibit high mechanical properties in their as-spun as well as heat-treated forms.
Thus it is an object of the present invention to provide a process for forming uniformly oriented high denier LCP filaments. The high denier filament means a filament of higher than 50 dpf
It is also an object of the present invention to provide a process for forming high denier LCP filaments of higher than 50 dpf, which exhibit enhanced mechanical, thermal and chemical resistance properties in the as-spun as well as heat-treated form.
It is further an object of the present invention to provide a process for forming high denier LCP filaments, which exhibit properties comparable to those of low denier LCP filaments (i.e., filaments of less than 10 dpf) in their as-spun as well as heat treated states.
It is also an object of the present invention to provide high denier LCP filaments of higher than 50 dpf having properties comparable to those of low denier LCP filaments of less than 10 dpf
Finally, it is an object of the present invention to provide a cost-effective, industrially economic way to heat-treat the high denier filaments of this invention directly on the bobbin to produce high denier filaments of superior mechanical and physical properties.
There is high desirability in forming uniformly oriented high denier LCP filaments, which exhibit enhanced mechanical, thermal and chemical resistance properties in the as-spun as well as heat-treated form. For example, high denier LCP filaments can replace steel wires in steel belted tires. Furthermore, since LCP filaments are of substantially lower density when compared with steel wires, LCP filaments are expected to feature much superior properties than that exhibited by the steel wires. It is further obvious from the following prior art that there is a real need for high denier LCP filaments that exhibit enhanced mechanical, thermal, and chemical resistance properties.
Prior Art
The following references are disclosed as background prior art.
U.S. Pat. No. 4,183,895 describes a process for treating anisotropic melt forming polymeric products. A process of heat treatment obtained the fibers having enhanced mechanical properties and the fiber tenacity was increased by at least 50% and to at least 10 grams per denier.
U.S. Pat. No. 4,468,364 teaches a process for extruding thermotropic liquid crystalline polymers (LCPs). It is claimed that extrusion of an LCP through a die orifice having an L/D ratio of less than 2 (preferably 0), and at a draw-down ratio of less than 4 (preferably 1), one can obtain filaments featuring high mechanical properties.
U.S. Pat. No. 4,910,057 describes a highly elongated member of substantially uniform cross-sectional configuration, which is capable of improved service as a stiffening support in an optical fiber cable.
U.S. Pat. No. 5,246,776 teaches an aramide monofilament and method of making the same.
U.S. Pat. No. 5,427,165 describes a reinforcement assemblage formed at least in part of continuous monofilaments of liquid crystal organic polymer(s). The polymers used therein are primarily aramids.
Japanese laid open Patent No. 4-333616 teaches a method of manufacturing filaments of 50 to 2000 dpf from molten liquid crystalline polymers. The heat-treated mechanical properties of these filaments were significantly inferior than the properties reported for the corresponding lower denier filaments of 5 to 10 dpf.
J. Rheology 1992, Vol. 36 (p. 1057-1078) reports a study of the rheology and orientation behavior of a thermotropic liquid crystalline polyester using capillary dies of different aspect ratios.
J. Appl. Polym. Sci. 1995, Vol. 55 (p. 1489-1493) reports orientation distribution in extruded rods of a thermotropic liquid crystalline polyesters. The orientation function increases with increasing apparent shear rate from 166 to 270 sectxe2x88x921, but decreases with increasing apparent shear rate from 566 to 780 secxe2x88x921.
All of the references described herein are incorporated herein by reference in their entirety.
Unexpectedly and surprisingly it has now been found that both as-spun and heat-treated high denier filaments of at least 50 denier per filaments can be made that feature essentially uniform molecular orientation across the cross-section. Furthermore, these high denier filaments feature remarkably good tensile properties retaining at least 80 to 90 percent of the properties expected of conventional low denierxe2x80x945 to 10 dpf-filaments, which was hitherto unattainable by any of the known prior art references as briefly described hereinabove.
Thus, in accordance with this invention there is provided a process for forming an as spun filament of a thermotropic liquid crystalline polymer having the following properties:
(i) denier of at least about 50 denier per filament;
(ii) tenacity of at least about 8 grams per denier;
(iii) modulus of at least about 450 grams per denier; and
(iv) elongation of at least about 2 percent.
The process of the present invention is comprised of the following steps:
(a) heating a thermotropic liquid crystalline polymer to a temperature of at least about 15xc2x0 C. above its melting transition to form a fluid stream of said thermotropic polymer;
(b) passing said stream through a heated extrusion chamber, wherein said chamber is disposed with a suitable cylindrical orifice to form the filament of said polymer, and wherein said cylindrical orifice has an aspect ratio of length to diameter (L/D) greater than about 1 and less than about 15; and
(c) winding said filament at a take-up speed of at least about 200 meters per minute and draw-down (DD) ratio of at least about 4; and with the proviso that when L/D is between 0 to 2, the DD is at least 4 so as to form the filament of essentially uniform molecular orientation across its cross-section and having a denier of at least about 50 denier per filament.
In another aspect of the invention there is also provided a process for forming a heat-treated filament of a thermotropic liquid crystalline polymer having the following properties:
(i) denier of at least about 50 denier per filament;
(ii) tenacity of at least about 20 grams per denier;
(iii) modulus of at least about 600 grams per denier; and
(iv) elongation of at least about 3 percent.
Thus in accordance with this aspect of the present invention, the process is comprised of the following steps:
(a) heating a thermotropic liquid crystalline polymer to a temperature of about 15xc2x0 C. to about 50xc2x0 C. above its melting transition to form a fluid stream of said polymer;
(b) extruding said stream of polymer through a heated cylindrical spinneret having at least one extrusion capillary to form a filament, wherein said capillary has an aspect ratio of length to diameter (L/D) in the range of from about 1 to about 10;
(c) winding said filament at a take-up speed of at least about 200 meters per minute and draw-down ratio of from about 5 to about 40 so as to form a filament of essentially uniform molecular orientation across the cross-section and having a denier in the range of from about 50 to about 1000 denier per filament; and
(d) heat-treating said filament at suitable temperature and pressure conditions for a sufficient period of time, optionally in the presence of an inert atmosphere, to form the heat-treated filament.
In yet another aspect of this invention there is also provided an as-spun filament of a thermotropic liquid crystalline polymer.
In a further aspect of this invention there is also provided a heat-treated filament of a thermotropic liquid crystalline polymer.
In another facet of this invention there is also provided a process for heat treating the high denier filaments of this invention directly on the bobbin on which they were wound while spinning.
Other aspects and advantages of the present invention are described further in the following detailed description of the preferred embodiments thereof.
Examples of the aromatic-aliphatic polyesters and polyesteramides which may be used in practicing the invention may include those having the following structures.
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